CHESTER, Pa. — First, it was a 19-year-old who made his MLS debut, starting the opening game of the season at center back on March 3. At the end of that month, an 18-year-old joined him on the backline. And last week, midway through the Philadelphia Union’s 2-0 loss to Orlando, another 19-year-old was invited to the defensive party, turning what had already been the youngest backline in MLS history into a unit that was probably even younger than most college teams.

And now, due to injuries to defensive starters Fabinho and Jack Elliott, that teenage trio of Auston Trusty, Matthew Real and Mark McKenzie may all start together when the Union hit the road to face FC Dallas on Saturday (8 p.m., PHL17) — just as it was a few years ago when they were teammates on the club’s youth academy teams.

“It’s a surreal moment because you play U-14, U-16 together, hang out with them on trips, travel with them when you’re 15, 16 years old,” McKenzie told The Athletic...

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